Second-time-around echo immune radar system

ABSTRACT

A radar system for eliminating second-time-around echoes while simultaneously providing protection against radar &#39;&#39;&#39;&#39;blind speed&#39;&#39;&#39;&#39; problems. The invention provides two radar systems utilizing a common antenna and operating at two different frequencies and at two different pulse repetition rates. The video outputs from each system are combined to yield the target display.

United States Patent represented by the Administrator of the FederalAviation Administration SECOND-TlME-AROUND ECHO IMMUNE RADAR SYSTEMPrimary Examiner-Malcolm F. Hubler AuorneysCharles K. Wright, Jr.,William G. Gapcynski and Lawrence A. Neureither 3 Claims 2 Drawmg FigsABSTRACT: A radar system for eliminating second-time- U.S. Cl 343/7],around echoes while simultaneously providing protection 343/6 TV againstradar "blind speed" problems. The invention provides Int. Cl G015 7/06,two radar systems utilizing a common antenna and operating G015 9/42 attwo different frequencies and at two different pulse repeti- Field ofSearch 343/7.7, 6 tion rates. The video outputs from each system arecombined TV to yield the target display.

DIPLEXER l 2 6 Q CHANNEL A 5 CHANNEL 5 RADAR PRF RADAR SYSTEM SYSTEMSYNCHRDNIZER CRYSTAL CRYSTAL PATENTEU NUVP. I971 SHEET 1 0F 2 DIPLEXER 16 CHANNEL A CHANNEL 8 j RADAR RADAR SYSTEM SYSTEM 7 K CRYSTAL V'DEOCRYSTAL REALIGNMENT AND COMBINER VIDEO DISPLAY SYSTEM PRF SYNCHRONIZERINVENTOR THOMPSON J. SiM PSON, SR.

A ()RNEY PATENTED HUVZ I971 CHANNEL A RADAR SYSTEM CRYSTAL SHEEI 2 [1F 2DIPLEXER l 2 CHANNEL 8 RADAR PRF SYSTEM SYNCHRONIZER CRYSTAL cAMERA T.v. REPEATER s'cdPEs FIG.2

I N VENTOR THOMPSON J. SIMPSON, SR.

ATTORNEY SECOND-TIME-AROUND ECHO IMMUNE RADAR SYSTEM The inventiondescribed herein may be manufactured and used by or for the Governmentfor governmental purposes without the payment to me of any royaltythereon.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention isan improved radar system which removes second-time-around echoes from aradar display while also providing protection against radar blind speedproblems.

2. Description of the Prior Art The secondtime-around echo occurs whenan extremely large fixed target such as a mountain range or largethunderstorm exists at a radar range just beyond the normal listeningperiod of the radar receiving system. Second-time-around echo normallyaffects short-range radar systems such as an approach control facilitywhere the maximum radar range is usually limited to 40 to 60 miles. Aconsideration of the operation of a typical short-range radar systemwill show how the second-time-around radar problem arises. Consider ashortrange radar system whose listening period is limited to 50 miles.After the transmitter is fired for an appropriate period of time, thesignal package travels outbound from the radar antenna at the velocityof light, during which time the receiver is in a listening mode. Whenthe pulse package is 50 miles distant from the radar antenna, thereceiver listening period is cut off, the transmitter is tired again,and the entire sequence is repeated, the rate of repetition beingdefined as the pulse recurrence frequency (PRF) of the system.Meanwhile, the first pulse package continues its outbound travel and ata range of 70 miles assume that it illuminates a large mountain. At thetime the first pulse package intercepts the mountain, the receiver hasbeen in its second listening period for approximately miles of range.The energy returned from the mountain, if it is of sufficient magnitude,will indicate a target in the second listening" period of the receiverat the false range of 20 miles from the radar. Such targets aredifficult to distinguish from similar targets which are in proper rangeregistration. .Familiarity with the surrounding site terrain andbehavior of the specific radar are to date the best means of determiningif the target is real orfalse.

The chief degrading effect of second-time-around echoes is that they arenot responsive to Doppler Moving Target Indicator (MTI) processing,hence they appear as large, annoying patches of clutter, which canobliterate target information. The reason that second-time-around echoesare not removed by MTI processing is because there is no signalcoherence from one listening period to the next in the case of magnetronradar transmitters because of the random phase firing of magnetrons. Acoherent radar transmitter, such as a crystal-controlled klystron, canbe employed and MT] processing can be utilized to eliminate thesecondtime-echo; however, such a system immediately becomes relegated toa single value PRF which the system vulnerable to blind speeds, aphenomenon unique to Doppler MTl radar systems. The blind speed existsin an MTl system under conditions where a moving target has a radialvelocity with respect to the radar such that the target moves onecomplete wavelength of the transmitting frequency during one PRFinterval.

in conventional approach radar systems the blind speed" problem iscircumvented to a great extent by staggering the PRF, that is, the PRFchanges value from one pulse interval to the next. Utilization ofintrachannel PRF staggering to avoid the "blind speed" problem will,where a coherent radar system is used, destroy the coherence from onelistening period to the next and will thus render the coherent systemvulnerable to the second-time-around echo. The need for staggered PRF tocombat the blind speed problem and the maintenance of frequencycoherence to prevent second-timearound echoes thus constitutes anincompatible requirement.

SUMMARY OF THE INVENTION This invention maintains frequency coherenceand permits the use of a staggered PRF, thus preventing blind speed"problems and second-time-around echoes. This is accomplished by usingtwo coherent radar channels, each operating on a different wavelengthand at a different PRF. Staggering will now be accomplished betweenindividual channels rather than within one specific channel. A PRFsynchronizer. is provided to establish a predetermined cyclic repetitionbetween the two PRFs so that the video outputs of the two channels maybeproperly realigned and to prevent both transmitter channels fromfiring at the same time.

This invention thus provides for coherence from one listening period tothe next, thereby permitting the MTl circuitry of each channel tooperate effectively to remove second-timearound echoes. Since eachchannel is operating on a singlevalue PRF to maintain coherence and thevalues of the PRF for each respective channel are different, when ablind speed occurs in one channel, the other channel will not have thesame blind speed, hence a video output-will be maintained and no loss ofsignal information will occur. The use of two radar channelsautomatically gives rise to frequency diversity which improves theprobability of target detection. Also since two radar channels are used,the reliability of the overall system is improved, and failure of onechannel can be readily noticed and repaired.

Accordingly, it is an object of this invention to provide a radar systemwhich eliminates second-time-around echoes and prevents loss of signalinformation due to the blind speed effect.

It is a further object of this invention to provide a radar system inwhich the probability of target detection is increased.

It is also an object of this invention to provide a radar system inwhich the reliability of the system is greatly increased.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are each functionalblock diagrams of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention comprises twoseparate crystal-controlled radar systems 1 and 2 operating on twodifferent carrier frequencies and using two different pulse recurrencefrequencies (PRF). The systems are connected through a diplexer 3 to acommon antenna 4, and PRF synchronization means 9 are provided to insurethat both transmitters do not fire at the same time. The receiver videooutputs 5, 6 of FIG. 1 are connected to a video realignment andcombining means 7, and the combined video output from 7 is applied to avideo display system 8 to obtain a usable video display. Each radarsystem is provided with MTI processing circuitry to removesecondtime-around echo since each radar system is coherent from onelistening period to the next.

The video realignment and combining means 7 serves to properly align thevideo information from each channel so that the two video signals can becombined and presented to a display system. Another embodiment of avideo realignment and combining means is shown in FIG. 2. FIG. 2illustrates a system identical with that shown in FIG. I except that ameans for optically realigning andcombining the video outputs from thetwo radar systems is shown. The realignment and combin ing meanscomprises two conventional cathode-ray display scopes 10, 11 connectedto video outputs 5,6 respectively and positioned so that the scope facesare adjacent and perpendicular to one another, a half-silvered mirror 12positioned so that light from the scopes 10, ll strikes the oppositesides of the mirror at a 45 angle, and a conventional televisioncameral3 and display scopes I4.

The output from each respective radar channel is recorded on itsassociated scope or 11. This scope display presents the targetinformation in aligned form, and the video outputs are combined throughthe half-silvered mirror 12 and are displayed through a conventionaltelevision camera 13 and its associated receiver scopes 14. Thisrepresents only one method of achieving realignment and combination, andothers would be obvious to one skilled in the art. For example, thevideo information from each radar system could be converted to digitalform, stored in memory, read out at the proper time for realignment andadded to obtain a combined video output. Alternatively, each videosignal could be converted to optical form, scanned by a televisioncamera, and the resulting electrical signals added to obtain thecombined video signal.

I claim:

I. A radar system for eliminating second-time-around echoes and blindspeed problems comprising a first coherent radar system employing MovingTarget indicator circuitry and operating on a first carrier frequencyand a first pulse recurrence frequency, a second coherent radar systememploying Moving Target Indicator circuitry and operating on a secondcarrier frequency and a second pulse recurrence frequency, a commonantenna system, means for realigning and combining the video outputsignals from said first and second radar systems to provide a combinedvideo signal, and means to display said combined video signal.

2. The device of claim 1 in which synchronization means are provided toprevent simultaneous firing of said first and second radar systems.

3. The device of claim 1 in which said video realigning and combiningmeans comprises a first cathode-ray display scope displaying the videooutput from said first radar system, a second cathode-ray display scopedisplaying the video output from said second radar system, said firstand second display scopes positioned so that the faces of scopes are inperpendicular relationship to a half-silvered mirror positioned so thatlight from each of said first and second display scopes strikes oppositesides of said mirror at a 45 angle, a television camera adapted todisplay the video output from said television camera.

1. A radar system for eliminating second-time-around echoes and''''blind speed'''' problems comprising a first coherent radar systememploying Moving Target Indicator circuitry and operating on a firstcarrier frequency and a first pulse recurrence frequency, a secondcoherent radar system employing Moving Target Indicator circuitry andoperating on a second carrier frequency and a second pulse recurrencefrequency, a common antenna system, means for realigning and combiningthe video output signals from said first and second radar systems toprovide a combined video signal, and means to display said combinedvideo signal.
 2. The device of claim 1 in which synchronization meansare provided to prevent simultaneous firing of said first and secondradar systems.
 3. The device of claim 1 in which said video realigningand combining means comprises a first cathode-ray display scopedisplaying the video output from said first radar system, a secondcathode-ray display scope displaying the video output from said secondradar system, said first and second display scopes positioned so thatthe faces of scopes are in perpendicular relationship to a half-silveredmirror positioned so that light from each of said first and seconddisplay scopes strikes opposite sides of said mirror at a 45* angle, atelevision camera adapted to record the image from said half-silveredmirror, and means to display the video output from said televisioncamera.